The conventional technology usually increases the processing speed of central processing unit (CPU) chips by elevating an operation voltage or overclocking. However, such method for increasing speed is only effective to CPU chips. For the conventional memory devices, the internal supply voltage of the memory devices remains at a constant value regardless of variation of the operation voltage being applied. This is to say that the processing speed of the memory devices remains the same. Therefore, it is not feasible to make use of an external signal of the memory devices to adjust the internal supply voltage. Consequently, the processing speed of the memory devices cannot be increased.